It is known in radio frequency systems to use a coaxial or waveguide transmission line to propagate radio frequency (RF) signals from a signal source, such as from a broadcast transmitter to a broadcast antenna. A disadvantage of systems in current use is that a waveguide or transmission line can deteriorate gradually due to its exposure to a number of destructive factors, while the deterioration can continue undetected in normal use until a catastrophic failure reveals one or more problems. Since a transmission line may be used, for example, for broadcast of several communication channels using a shared, broad-band antenna system, it is possible for a multiplicity of channel transmissions to be rendered inoperable or degraded to a significant extent until repair parts can be procured and installed.
Destructive factors likely to contribute to harm to transmission lines over extended time periods can be, for example, galvanic corrosion; cold flow under bolts caused by repeated temperature swings; metal fatigue associated with repeated mechanical stress cycling; exposure to chemically reactive pollutants; exposure to airborne salt; exposure to water condensed inside the transmission line due to temporary loss of pressurization or dehumidification; and hidden workmanship errors such as pinched O-rings or pinhole flaws in the materials from which the transmission line is constructed.
These and many other factors can lead to significant increases in voltage standing wave ratio (VSWR), high voltage breakdown, and other faults demanding immediate attention. VSWR increases stemming from such degradation can manifest suddenly due to events such as storm stress on deteriorated materials, or can manifest gradually in the form of cumulative degradation in performance over a protracted period. Both problem modalities can require extensive efforts simply to identify and localize a single-point failure even after it has occurred. Multiple-point degradations can likewise become established, requiring prolonged and potentially ineffective troubleshooting.
A typical operator response to prevent the escalation of some of these problem modalities is the reduction in output power, resulting in loss of reception for the furthest customers and degraded reception for those nearer. Even this response is not diagnostic, so it is still necessary to identify the failure that caused the system fault. A time consuming diagnostic procedure can ensue, leading to the development of a corrective action plan, which may involve shutting down a multi-channel broadcast facility for an extended period during prime viewing hours, switching over to a reduced-performance backup system for an indefinite servicing period, or another method, any of which can potentially entail significant contract violations and incur financial penalties.
Accordingly, there is a need for a diagnostic apparatus and method that can be applied non-intrusively to provide detection of degradation taking place in a transmission line.